The present invention relates to prostheses in general, and more particularly to improvements in internal prostheses, especially those which can replace knee joints.
It is known to assemble an internal prosthesis of two components having extensions which can be embedded in the marrow cavities of bones whose joint (such as a knee joint) is to be replaced with a prosthesis. It is also known to construct a prosthesis in such a way that the two components have a freedom of movement relative to each other in the longitudinal direction of the components, e.g., to increase or reduce the distance between a femur and a tibia. In addition, in the case of a knee prosthesis the tibia can pivot relative to the femur and/or vice versa about a transverse axis and the tibia can turn relative to the femur about a second axis which is at least substantially normal to the pivot axis and extends longitudinally of the tibia. The extent to which the tibia can move longitudinally toward or away from the femur (or vice versa) is determined or is intended to be determined by the so-called soft-part apparatus to the extent that such apparatus is capable of carrying out its function. This means that the movability of the tibia relative to the femur in the longitudinal direction of the tibia can be limited only if the aforementioned apparatus is capable of performing such work. On the other hand, there exists an urgent need for highly versatile internal prostheses, e.g., for prostheses to be used in lieu of knee joints, which allow for longitudinal movements of the articulately connected bones relative to each other irrespective of the condition of that portion or those portions of the anatomy which are presently intended to control and determine the extent of such movements.